Age hardening austenitic alloy steels



Patented Sept. 26, 1950 UNITED. STATES} PATENT mm AGE HARDENING AUSTENITIC ALLOY STEELS Peter PaysolnpNew York, N. Y., assignor to Crucible Steel Company of America, New York, N. Y., a corporation of New Jersey No Drawing.

room and elevated temperatures ranging up to 1350 F; or more. The steel ofthe' invention is eminently suitable for use in automotive engine valves and valve parts, particularly the exhaust valves and valve parts thereof. 7

Although the heat hardening steel of my patent, Reissue 20,421, has-been outstandingly successful for exhaust valves in automotive engines, it has the disadvantage of being relatively expensive owing to its high alloy content and high fabricating cost. Also, its resistance to stretching at elevated'temperatures and its corrosion resistance in molten lead oxide are not as high as de-' sired. In my Patent 2,484,903, I have described a further type of heat hardening steel suitable for automotive engine valves and valveparts, which is lower in alloy content, hence lower in cost, than the steel of my aforesaid patent, and which also is somewhat higher in corrosion resistance in molten lead oxide and higher in resistance to stretching at elevated temperatures than the steel of my patent, but, which, nevertheless, leaves something to be desired as to the latter characteristic.

Steels which are most resistant to stretching at the elevated temperatures to which automotive engine valves are subjected, ranging up to about 1350 F., are those possessing an austenitic structure. The heat hardenable steels are somewhat inferior by comparison in this respectbecause in the heat hardened state, they contain a considerable proportion of the sigma phase, 5' which is a relatively hard and brittle constituent, 1

and hence is less effective than austenite in imparting elevated temperature stretch resistance in steel. On the other hand, the austenitic steels are in general deficient in two vital respects as Application November 2, 1949, Serial No. 125,173"

8 Claims. (01. 124) r 2 'v in the stem of the valve, and they are not hard enough at the elevated temperatures encountered in the automotive engine exhaust stream for wear resistance at the head, or seating surface,

of the valve.

To overcome the first deficiency, it has been the, practice to weld a stem of an ordinary hardenablesteel, such as A181 8640, to an austenitic steel head, and then to harden the stem end by .-heat treatment. Thisinvolves a rather expensive resistance welding manufacturing operation. In some cases the stem and tappet end 7 of a solid austenitic valve are hardened by cold working, but this again is an expensive manufacturing operation.

To overcome the second deficiency, it has been the practice to weld a deposit, of a hard facing material, such as a high carbon, cobalt-chromium-tungsten alloy, to the seating surface of the valve. This is so expensive an operation b cause of the high cost of both the hard facing material and the welding operation, that it is justifiable only for aeronautical, bus and truck engine valves.

The ideal material for exhaust valve service would be an austenitic steel of relatively low cost which could be hardened b heat treatment.

The-usual method of martensitic hardening of known, very few are'hardenable to the degree required for valve steel service. It has been determined by many yearsv of experience. that for adequate service, the tappet end of an automotive engine exhaust valve should have a hardness of about Rockwell 038, or higher.

Some austenitic steels containing chromium,

nickel, manganese and aluminum, which harden by precipitation to at least Rockwell C38 are described in my application, Serial No. 792,929, filed December 20, 1947; Serial No. 63,200 filed December 2, 1948 and Serial No- 120,412 filed October 8, 1949,'-but all of these contain nickel over about 19%, and it is desirable from the economic viewpoint that nickel beheld to a minimum in a steel forautomotive exhaust valves.

Now I have discovered that by employing and not have the wholly austenitic structure desired. On the other hand, since the hardness does not increase with increasing nickel above 14%, it is desirable from the economy viewpoint to hold the nickel to a maximum of about 16%.

The effect of chromium on the hardening of the steel may be seen from th test results in Table II.

, Table II.Efiect of chromium on hardening All samples solution treated at 2300 F. and aged 16 hours at the temperatures shown.

. Rockwell O Hardness Analysis, Per Cent on Aging Bar Bal.

C Mn Si Ni Cr A1 1200' F 1300 I. 1400 F.

2037.. .44 4.5 .53 14.7 8.6 3.2 Fe.. 12 16 21 2251 .48 4. 7 .63 14. 7 10.3 3.3 Fe 21 37 34 2254 .50 4.8 .66 14. 6 11.5 3. 3 Fe 38 36 2257 .52 4. 8 70 14. 6 13.6 3. 3 Fe. 35 39 37 2121.. 52 4. 8 68 14. 2 14. 9 3. 4 Fe." 33 42 38 1350 F., under a stress of 1000 p. s. i.; and high hardness at exhaust valve service temperatures, as will be shown subsequently.

The steel of this invention has the following ranges of composition for "broad and preferred analyses respectively:

Broad Range Preferred Range Carbon about .50

13 .0 Chromium about 10.00 to l5.00% about 12.00 to 14.0 Aluminium. about 2.50 to 4.00% about 3.00 to 3.50 Balance Substantially Iron Substantially Iron.

the steel. about 0.3%, and any or all of such elements as vanadium, tungsten, molybdenum, cobalt and copper up to about 5% in aggregate.

In addition to the elements above specified, the

These data show a marked increase in hardening of the 10.3% Cr steel compared with the 8.6% Cr steel, but only a very gradual increase in hardening in the steels ranging from 10.3% to 14.9% chromium. Therefore, the chromium in the steel of this invention should be at least about 10% but there is not much advantag in raising the chromium beyond about 15%, and from the viewpoint of economy about 12 to 14% is sufiicient.

A minimum of about 2.5% manganese is required in the steel of the invention to assure that after it is aged it will be austenitic, that is, substantially non-magnetic. The hardening of the steel is not much aiTected by increasing the manganese beyond about 3.5% as shown by the test results in Table III;

Table [IL-Effect of manganese on hardening All samples solution treated at 2300 F. and aged 16 hours at the temperatures shown.

41C], Ana1ys1s, Per Cent gif i g l Bar Bal.

0 M11 81 N1 01 A1 1200 F. 1300 F. 1 100 F.

2349.--- .50 3.5 .87 14.9 12.8 2.9 Fe... 32 39 32 2350.--- 53 4. 8 90 14. 7 12. 7 3. 0 Fe. 36 41 35 2351.- .53 6. 2 .86 15.3 12.5 3.0 Fe 39 41 36 2352--" .54 7.8 .42 14. 9 12.4 3.0 Fe 39 43 35 2353;--- 53 9. 6 14. 8 12. 8 3.1 Fe. 41 42 36 Thus nitrogen may be included up to When the nickel is under about 13% the steel does not harden adequately as may be seen from the test results in Table I below:

Table I .E17ect of nickel on hardening All samples solution treated at 2300" F. and

aged 16 hours at the temperatures shown.

Since manganese is not an expensive element I may use up to about 10% in the steel of the invention. However, the forceability of the steel becomes more difficult as the manganese is increased and I therefore prefer to keep the manganese at a maximum of about 6%.

Aluminum has a very important effect on the hardening of the steel because it is probable that the age hardening is caused by the precipitation of an aluminum compound. But the steel must contain an appreciable amount of aluminum, that Rockwell C Hardness Analysis, Per Cent on Aging Bar Ba].

0 Mn Si Ni Cr Al 1200 F. 1300 F. 1400 F.

2068.. .47 4. 7 .61 9. 7 13. 2 3.1 Fe 20 23 24 -2071 .47 4.7 .62 12.1 13.2 3.2 Fe 19 28 32 5981-- .51 5.3 75 13.9 13.9 3.1 Fe 38 41 38 2257 52 4. 8 14.6 13. 6 3. 3 Fe 35 39 37 Furthermore, the steels with less than about 13% Ni are somewhat magnetic after the high temperature solution treatment and therefore do is, over about 2.5%, in order for the steel to develop adequate hardness for exhaust valve service, as may be seen from the test results in Table IV.

Table IV.--E17ect of aluminum on hardening All samples solution treated at 2300 F. and aged for 16 hours at the temperatures shown.

this" changed: the: hardness of s'everal'f0.15% C austenitic steels from Rockwell B90 to only B95 (Rockwell C9 to C16). It was therefore. quite unexpected. to find that carbon had the marked,

Since aluminum has a detrimental elfect on the forgeability-of the steel, I limit the aluminum content to a maximum of about 4%.

Silicon has a minor but definite effect on the hardening, as shown by the test results in Table V, and the steel should, therefore, contain a minimum of about 0.20% silicon. For satisfactory effect on the hardness of the precipitation.

hardening steels of this invention as shown by the test results in Table VI below.

Table VI.--E1fect of carbon on hardening All samples solution treated at 2300 F. and aged for 16 hours, at the temperatures shown.

n n Analysls, Per Cent g f i g g i s Bar Bal.

C Mn Si Ni Cr A1 1200 F. 1300 F. 1400F.

2250.-.. 35 4. 5 62 14. G 10.0 3. 4 18 23 23 2251.--- 48 4. 7 63 14. 7 10. 3 3. 3 21' 37 34 2252.--- 63 4. S 64 14. 7 9. 7 3. 5 33. 4O 39. 2253 35 4. 8 67 14. 6 11. 8 3. 3 21 31." 25 i 2254.-" .50 4. s .66 14. 6 11.5 3. 3 25 3s 36 2255 62 4. 7 68 14. 7 11.6 3. 4 35 39 39" 2256... 35 4. 8 65 14. 7 13. 7 3. 3 23 34 2257 52 4. 8 70 14. 6 13. 6" 3. 3 35 39 37 2258.. 67 4.8 70 14. 7 13. 4 3. 3 33 42 39 2119. 33 4. 8 63 14. 0 14. 6 3. 4' 28 36 31 2l20 .37 4. 8 .59 14. 0 14. 5 3. 2 29 37 31 2122. 47 4. 8 68 14. 2' 1'4. 9 31 4 33' 42 38 forgeability, I limit the silicon in this steel to a maximum of about 2.5%, and preferably to 1.5%.

Table V.-E17ect of silicon on hardening All samples solution treated at 2300 F.' and aged for 16 hours at the temperature shown.

It is evident from these data that the steel of.

this invention should have a minimum of about 0.40% carbon inord'er to harden to about Rock Well C38 after solution and aging treatments, and preferably a carbon content over 0 .50%. However, in order to avoid difilculties in forging 1' Finally, I have discovered that the carbon content of the steel of this invention is a very important factor in its hardening. Ordinarily, carbon is not considered of importance in precipita tion hardening steels. It is, of course, known that carbon combines with many elements in steel to form carbides which have different degrees of solubility in austenite and which therefore can be dissolved and precipitated more or less at will by controlling the heating and the cooling of the steel. However, the precipitation of carbides in austenitic steel has very little effect on the hardness of the steel. For example, as stated in my paper, Changes in Austenitic Chromium-Nickel Steels During Exposures at 1100 to 1700 F., published in Trans. A. S. M. vol. 39, 1947, a heating for 200 to 900 hours at 1400 F. following a solution treatment at 2100 F. caused appreciable carbide precipitation but tabllshed that the steel of this invention need not shown in Table VII.

be solution treated at 2300 F. in order to behaidened. For example, the steel may be hardened from the as forged condition, or'after a solution treatment at 2200 F., but the hardening after a 2100 F. treatment is not quite satisfactory as Table VII.E17ect of solution treatment on hardening Samples as forged and solution treated as indicated were aged for '16 hours at 1300 F.

8.- I claim:

1. A forgeable and machinable, age-hardenable, austenitic alloy steel which, on solutiontreating at about 2300 F., followed by aging at Analysis, Per Cent A Solution Treated At s Bar Forged Mn Si Ni C1: A1 2100 F. 2200 F. 2300 F.

5979 51 3. 9 73 14. 0 14. 0 3. 1 Fe 39 35 39 41 5981 51 5. 3 75 13. 9 l3. 9 3. 1 Fe 41 33 40 41 Evidently the hardening of the steel may depend on some cold working combined with some about 1300 F., is characterized in having a hard ness in excess of C 35 Rockwell, said steel consolution of precipitating compounds (the as 15 taining: about 0.4 to 0.8% carbon; about 13.5 forged condition) or on a high degree of solution to 16% nickel; about 3.5 to manganese; of the compound (the 2200 F. and 2300 F. soluabout 10 to chromium; about 0.2 to 2.5% tion treatments). The treatment at 2100 F. apsilicon; about 2.5 to 4% aluminum; up to about parently relieves the cold worked condition and 0.3% nitrogen; up to about 5% in aggregate of does not efiect sufiicient saturation to cause ade- 20 other elements which do not impair the age-hardquate hardening during the subsequent aging. ening properties of the steel; and the balance As may be seen from the tables above, the opsubstantially all iron.

timum aging temperature for the steel of this 2. A forgeable and machinable, age-hardeninvention is about 1300 F. but in many instances able, austenitic alloy steel which, on solutionadequate hardness for valve service is also obtreating at about 2300 F., followed by aging at tained by an aging treatment of 1400 F. about 1300 F., is characterized in having a, hard- In spite of the low chromium content in the ness in excess of C Rockwell, said steel consteel of this invention this steel has satisfactory taining: about 0.5 to 0.6% carbon; about 14 to 15% resistance to corrosion in molten lead oxide as nickel; about 4.5 to 6% manganese; about 12 to may be seen in Table VIII. 30 14% chromium; about 0.5 to 1.5% silicon; about Table VIII.Corrosion resistance in lead oxide at 1675 F.

Analysis, Per Cent Bar e als 0 Mn s1 Ni Cr Al M6 Of the steels in Table VIII, the first three represent this invention; the fourth is a commercial valve steel of the sigma phase type, per Payson U. S. Patent Re. 20,421; and the fifth is a commercial valve steel of the martensitic type.

The stretch resistance of the steel of this invention is far superior to that of. the two commercial valve steels mentioned above. As solution treated and aged to a hardness of Rockwell C38 to 40, the steel of this invention tested at 1350 F. with a stress of 10,000 p. s. i. elongated about 0.2 to 0.5% in 8 hours. Under these same testing conditions, the sigma phase valve steel represented by Bar 5829 of Table VIII elongated about 1.5% in 8 hours; while the martensitic valve steel represented by Bar 2011 of Table VIII stretched exceedingly and eventually ruptured in much less than 8'hours.

Finally, the steel of this invention as solution treated and aged to a hardness of Rockwell C38 to 40 has excellent hot hardness, having values of 215 to 255 Brinell at 1200 to 1400 F., in comparison with 175 to 200 Brinell for the sigma phase valve steel of Table VII; 120 to 150 for the martensitic type valve steel of Table VIII; and 105 to 140 for the austenitic 21% chromium-12% nickel type valve steel.

Thus on the basis of corrosion resistance, stretch resistance, hardness at elevated temperatures, as well as hardness at room temperature, the steel of this invention is eminently suitable for automotive engine exhaust valve service.

3 to 3.5% aluminum; up to about 0.3% nitrogen; up to about 5% in aggregate of other elements which do not impair the age-hardening properties of the steel; and the balance substantially all iron.

3. An age-hardened austenitic alloy steel having a hardness in excess of C 35 Rockwell, said steel containing: about 0.4 to 0.8% carbon; about 13.5 to 16% nickel; about 3.5 to 10% manganese; about 10 to 15% chromium; about 0.2 to 2.5% silicon; about 2.5 to 4% aluminum; up to about 0.3% nitrogen; up to about 5% in aggregate of other elements which do not impair the agehardening properties of the steel; and the balance substantially all iron.

4. An age-hardened austenitic allo steel having a hardness in excess of C 35 Rockwell, said steel containing: about 0.5 to 0.6% carbon; about 14 to 15% nickel; about 4.5 to 6% manganese; about 12 to 14% chromium; about 0.5 to 1.5% silicon; about 3 to 3.5% aluminum; up to about 0.3% nitrogen; up to about 5% in aggregate of other elements which do not impair the agehardening properties of the steel; and the balance substantially all iron.

5. A forgeable and machinable, age-hardenable, austenitic alloy steel which, on solutiontreating at about 2300 F., followed by aging at about 1300 F., is characterized in having a hardness in excess of C 35 Rockwell, said steel containing: about 0.4 to 0.8% carbon; about 13.5 to 16% nickel; and about 3.5 to 10% manganese;

9 I about 10 to 15% chromium; about 0.2 to 2.5% silicon; about 2.5 to 4% aluminum; and the balance iron.

.6. A forgeable and machinable, age-hardenable, austenitic alloy steel which, on solutiontreating at about 2300 F., followed by aging at about 1300 F., is characterized in having a hardness in excess of C 35 Rockwell, said steel containing: about 0.5.to 0.6% carbon; about 14 to 15% nickel; about 4.5 to 6% manganese; about 10 12 to 14% chromium; about 0.5 to 1.5% silicon; about 3 to 3.5% aluminum; and the balance iron.

7. An age-hardened austenitic alloy steel having a hardness in excess of C 35 Rockwell, said steel containing about 0.4 to 0.8% carbon; about 15 13.5 to 16% nickel; about 3.5 to 10% manganese; about 10 to 15% chromium; about 0.2 to 2.5% silicon; about 2.5 to 4% aluminum; and the balance iron.

5 12 to 14% chromium; about 0.5 to 1.5% silicon;

about 3 to 3.5% aluminum; and the balance iron.

PETER PAYSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

1. A FOREGEABLE AND MACHINABLE, AGE-HARDENABLE, AUSTENITIC ALLOY STEEL WHICH, ON SOLUTIONTREATING AT ABOUT 2300*F., FOLLOWED BY AGING AT ABOUT 1300*F., IS CHARACTERIZED IN HAVING A HARDNESS IN EXCESS OF "C" 35 ROCKWELL, SAID STEEL CONTAINING: ABOUT 0.4 TO 0.8% CARBON; ABOUT 13.5 TO 16% NICKEL; ABOUT 3.5 TO 10% MANGANESE; ABOUT 10 TO 15% CHROMIUM; ABOUT 0.2 TO 2.5% SILICON; ABOUT 2.5 TO 4% ALUMINUM; UP TO ABOUT 0.3% NITROGEN; UP TO ABOUT 5% IN AGGREGATE OF OTHER ELEMENTS WHICH DO NOT IMPAIR THE AGE-HARDENING PROPERTIES OF THE STEEL; AND THE BALANCE SUBSTANTIALLY ALL IRON. 